Workshop on Cretaceous Climate and Ocean Dynamics

Sea-level and climate fluctuations at the Cenomanian-Turonian Boundary of the Anglo-Paris Basin (Eastbourne) and European Tethyan margin (Cassis and Vergons, southeastern France)

The Eastbourne section, exposed along the seashore cliffs between Beachy Head and Eastbourne, Sussex, England, is about 13m thick, and spans the Cenomanian-Turonian (CT) boundary. It is one of the more expanded C-T transitions in England with a significantly higher sedimentation rate than the condensed sections from the Dover-Folkstone region. A sea-level change marks the base of the section with an abrupt change from the Grey Chalk to the Plenus Marl. This sharp, erosive and burrowed lithological contact is interpreted as a significant sequence boundary. The lower member of the Plenus Marls, including the one meter thick interbedded grey chalk, is considered as a lowstand deposits (SMW). The base of the upper member of the Plenus Marls is interpreted as a transgressive surface. Upsection, sedimentation becomes more calcareous and corresponds to a major rapid transgression. This interval corresponds to a transgressive system track, due to the presence of several calcarenitic layers with discrete harground surfaces and abundant Inoceramus prisms deposited by current winowing that indicate a period of starvation in the basin during the rising sea-level. The total organic content is very low, ranging from 0.02 to 0.25% of the sediment. Bulk rock and clay mineralogy indicate that the CT transition was characterized by an overall sea level highstand, interrupted by high frequency and high amplitude sea-level lows under cool-temperate and dry climate conditions. Positive values suggest short periods of overall oceanic cooling. These forced regressions may have been triggered by tectonism and/or glacio-eustasy.

The Cassis section is located near the town of Cassis at the l'Anse de l'Arène (France). The 95m thick sequence consists of basinal deposits, characterized by regularly cyclic alternation of pelagic limestone and marls. Similar to the Gun Garden section, the total organic carbon content is low (<0.7%). But the decrease in bioturbation observed during the R.cushmani and H. helvetica Zones indicates hypoxic conditions during the maximum of transgression.Compared to Eastbourne section, the Cassis section is characterized by higher accumulation rate. The W.archeocretacea Zone is therefore 50m thick (7m at Eastbourne). The two phased excursion observed at Eastbourne have been similarly observed at Cassis. The fauna suggests that the section is both shallow (middle neritic) and boreal. These conditions have favored a different foraminiferal assemblage that indicates a higher oxygen content for the W.archeocretacea Zone that at Eastbourne. At Cassis, sediments were deposited near the upper limit of the oxygen minimum zone.

The Vergons section is located in the eastern part of the Vocontian Basin (France), and is characterized by deeper paleo-environments with ryhthmically alternating marls and marly limestones layers. The top of R.cushmani Zone is marked by an erosive contact that suggests the presence of a significant hiatus. Above this hiatus, the W.archeocretacea Zone is characterized by deposition of the main black shale horizon (BSH) of the Thomel level, which consists of laminated layers enriched in organic matter (1-2%). The decimeter scale laminations of the BSH and the bioturbated organically depleted marls indicate that the oxygen minimum zone periodically reached the basin floor. Rhythmic deposition ends abruptly in the upper part of the laminated layer which is affected by significant slumping.

These sections indicate that paleogeographic features significantly influenced the distribution of organic rich sediments. However, high frequency climatic and sea level flucutations (including glacio-eustatic changes?) may be a promising alternative hypothesis to explain the Cenomanian-Turonian anoxic event.